1. Field of the Invention
The present invention relates to a high strength titanium alloy which has high strength, excellent weldability (i.e., ductility in heat affected zone (HAZ) after welding, the same meaning hereinafter) and good ductility to make the production of strips possible. The present invention relates to a titanium alloy coil-rolling process and a process for producing a coil-rolled titanium strip, in which the titanium is the above-mentioned titanium alloy.
2. Related Art
Titanium and its alloys are light, and excellent in strength, toughness and corrosion-resistance. Recently, therefore, they have widely been made practicable in the fields of the aerospace industry, the chemical industry and the like. However, titanium alloys are materials which are generally not so good in workability, so that costs for forming and working are very high, as compared with other materials. For example, Ti--6Al--4V, a typical .alpha.+.beta. type alloy, is a material which is difficult to work at room temperature. Thus, it is said that the alloy can hardly be made into a coil by cold rolling.
For this reason, at the time of rolling the Ti--6Al--4V alloy into a sheet form, a manner called pack-rolling is adopted. That is, the pack-rolling is a manner of stacking Ti--6Al--4V alloy sheets obtained by hot rolling in the form of layers, putting the sheets into a box made of mild steel, and hot rolling the sheets packed into the box under heat-retention for keeping its temperature more than a given temperature to produce a thin plate. In this process, however, a mild steel cover for making a pack and pack welding are necessary. Moreover, in order to block bonding of titanium alloy strips themselves, a releasing agent must be applied. In such a manner, the pack-rolling process requires very troublesome works and great cost, as compared with cold rolling. Additionally, the temperature range suitable for hot rolling is limited, to cause many restrictions in working.
On the contrary, Japanese Patent Application Laid-Open Nos. 3-274238 and 3-166350 discloses that the contents of Al, V and Mo in the parent material of titanium are defined and at least one alloying element selected from Fe, Ni, Co and Cr is comprised therein in an appropriate amount, so that a titanium alloy can be obtained which has a strength substantially equal to that of the Ti--6Al--4V alloy and are superior to the Ti--6Al--4V alloy in superplasticity and hot workability.
Japanese Patent Application Laid-Open Nos. 7-54081 and 7-54083 disclose a titanium alloy in which the Al content is reduced up to a level of 1.0-4.5%, the V content is limited to 1.5-4.5%, the Mo content is limited to 0.1-2.5%, and optionally a small amount of Fe or Ni is comprised thereinto, thereby keeping high strength and raising cold workability and weldability (in particular, HAZ after welding).
This titanium alloy has both cold workability and high strength, and further has improved weldability, and thus is an excellent alloy. However, in these inventions, flow-stress during plastic deformation is suppressed because of the necessity of ensuring excellent cold workability. Thus, its strength is considerably low. If the strength is raised, its cold workability drops. For this reason, production of cold strips are substantially impossible. Incidentally, in recent years, customers' demands of high strength and high ductility to titanium alloys have been becoming more and more strict. Thus, titanium alloys are desired to be improved still more.